(1) Field of the Invention
The present invention generally relates to internal combustion engines, and more particularly to an apparatus for diagnosing an abnormality in an electronic control type fuel injection system of an internal combustion engine. Further, the present invention is concerned with a fuel injection control system having such an apparatus.
(2) Description of the Related Art
In an internal combustion engine equipped with an electronic control type fuel injection system, a basic fuel injection period is calculated by using an intake manifold negative-pressure and an engine speed, or the amount of intake air and an engine speed. The basic fuel injection period thus obtained is then corrected by a feedback control process based on an output detection signal from an oxygen sensor fastened to an engine exhaust passage, so that a mixture of air and fuel supplied in an engine cylinder is always equal to a target air-fuel ratio, such as a stoichiometric air-fuel ratio.
Normally, upper and lower limit values are defined with respect to an air-fuel ratio feedback correction factor FAF used for correcting the basic fuel injection period by the feedback control process in order to prevent the basic fuel injection period from being excessively corrected. If a fault has occurred in the fuel injection system, for example, if a fuel injection valve cannot be closed and remains in the open state, the air-fuel ratio feedback correction factor FAF reaches the upper or lower limit. If this state is continuously maintained within a predetermined period, it is determined that a fault has occurred in the fuel injection system (see Japanese Laid-Open Patent Application 62-32237).
However, the air-fuel ratio feedback control process is not executed while the air-fuel ratio feedback correction factor remains equal to the upper or lower limit value. As a result, exhaust emissions will increase.
This problem will now be described in more detail. In the case where the air-fuel ratio deviates from the target air-fuel ratio, the exhaust emissions will increase in different ways depending on how the injection quantity is controlled at this time. More specifically, the degree of increase of exhaust emissions observed while the injection quantity is being controlled so as to increase or decrease these emissions (that is, the feedback control is being executed) is different from that observed while the injection quantity is fixed (open-loop control). This is due to the fact that the amount of exhausted oxygen changes as the injection quantity changes and thus the amount of oxygen in a catalyst increases or decreases. As a result, the exhaust gas can be reduced to some extent. In the case where the injection quantity is fixed, a state where no oxygen is contained in the catalyst or oxygen is excessively contained therein is continuously obtained. In such cases, it is impossible to reduce the exhaust gas.